The invention relates to a system controlling and measuring pulsed electromagnetic radiation, in particular to an auto-correlator.
Optical path lengths are deliberately altered in autocorrelators following beam splitting, for instance being controlled to periodically fluctuate, in order to measure pulse widths and pulse shapes using split beams. The technical background is especially significant in laser applications, foremost those using narrow light pulses less than 1 ns or 100 ps wide, which require knowledge of the pulse shape in the time domain. This pulse shape is ascertained by measuring the pulse autocorrelation function.
Typically a light beam is split into two parts through semi-transparent mirrors in interferometer-like manner in an autocorrelator, said two parts being time offset and then being combined in a non-linear, optical crystal, the optical summed frequency of the two split beam signals being formed in a detector. An illustrative design is disclosed in U.S. Pat. No. 4,628,473. The autocorrelation function of the delays in the individual beam delays is obtained by measuring the time-integral of the added-frequency signal, of which the intensity depends on the overlap in the time domain of the mutually time-shifted and/or space-shifted light pulse parts. In the design of the U.S. Pat. No. 4, 406,542 for instance, a rotating glass block is used to implement different delays in the two beam paths.
In general beam splitting is carried out in a Michelson interferometer comprising two semi-transparent mirrors and a geometric/mechanical element which time-shifts the generated split beams relative to each other. FIG. 2 shows the functional diagram as it is known from textbooks. Because the split beams first must be guided along separate paths and then are recombined, both costly precision mechanics and thin-film optics will be required. Adjustment entails substantial labor and moreover is hampered by the mandatory phase matching of the non-linear optical crystal. In palliation, the non-linearity function may be assumed by two-photon absorption in a photodetector and the time-integral of the photo-current may be taken as a function of the delays of the particular split beams, for instance in the manner described in xe2x80x9cAutocorrelation measurement of 6-fs pulses based on the two-photon-induced photocurrent in a GaAsP photodiodexe2x80x9d by J. K. Ranka et al in Optics Letters 22 (1997) #17, pp 1344-46.
The objective of the invention is to lower the costs of measuring the pulse shape, namely of auto-correlation and cross-correlation, and to lower the complexity of adjustments. Foremost it creates an auto-correlator of technically simple design which is manufactured in especially economical manner and which can be directly integrated for instance into ultra-narrow pulse laser systems.
The invention is based on a design to control and measure pulsed electromagnetic radiation, in particular collimated laser light, said light being split in beam receiver and then being recombined by a superposition or focusing element such as a lens and being picked up by a detector in the recombination zone. The beam receiver is at least in two parts; illustratively it consists of mutually adjacent beam profile splitters allowing altering the time-domain beam features of the split and preferably mutually parallel beams before they are recombined.
All time shifts and/or phase shifts are construed within the scope of the present invention being those entailing different delays in reflection or transmission, phase changes and/or changes in optical path lengths. The concept of spatial beam profile denotes the intensity distribution transversely to the direction of beam propagation, namely a function I(x,y), where x,y are the transverse coordinates, or I(r), where r is a coordinate running radially to the beam. Accordingly a beam profile splitter denotes a device changing the spatial beam profile of the incident light into split beams each with its own beam profile. The invention does not relate to conventional beam splitting without a change in profile, for instance in a partly transmitting mirror.
The manufacture of an autocorrelator of the invention only entails a fraction, for instance one tenth, of the price of conventional systems. The easily monitored and dimensionally stable design requires remarkably few accessories.
The beam profile splitters are independently controllable and as a result already a change by one of the beam profile splitters attains a change in the beam characteristics in the time domain.
The beam receiver is a system of mirror(s), plate(s), chamber(s) or crystal(s) comprising one beam profile splitter producing a reference split beam or leaving it unchanged, where at least one further beam profile splitter near the first one allows controlling the time characteristic(s) of the particular or any other split beam(s).
In another embodiment, the beam receiver is a mirror with a split specular surface of which the split mirror elements are mutually displaceable. A determinant advantage of the split mirror element design over conventional auto-correlators is the absence of any dispersion since no dispersive elements are needed. Preferably at least one split mirror element is displaceable in the direction of the specular normals relative to an illustratively stationary split mirror element or to a movable, phase-shifted split mirror element. In yet another embodiment, at least one of the beam profile splitters may be connected to an oscillator, for instance a loudspeaker, piezo oscillator or the like that is mounted to its back.
An alternative design is in the form of transmission elements acting as beam profile splitters and being rotatable relative to each other and/or about an axis subtending an axis with the incident beam, whereby optical path lengths and phase shifts may be implemented. In another design, two or more transmission elements are thin and flat, their (optical) thickness being made to match the pulse shape to be measured. Said elements may consist of glass, quartz, plastic or the like, they may also assume to form of strips or plates, and be mounted tightly near each other. These partly stationary and partly displaceable planar elements may be configured on edge or transversely in very simple, elegant and economical manner. At least one of the beam profile splitters may be driven by a motor and/or into vibration.
In another embodiment of the invention, the beam receiver for instance may be a cubic of parallelipipedic crystal comprising a segment which either is fitted with a terminal or lacks one and a further segment that is fitted with contact surfaces and in a state which can be changed by an electrical, a magnetic or an electromagnetic field in order that, illustratively, a phase shift be implemented by a change in the index of refraction. This design eliminates displaceable elements or modules; accordingly the autocorrelator shall be especially sturdy and easily handled.
The present invention moreover relates to an apparatus of the kind cited above wherein in particular collimated laser light shall be split in a beam receiver for the purpose of controlling and measuring pulsed electromagnetic radiation, said laser light being recombined beyond said beam receiver by means of a superposition or focusing device, for instance a lens, and shall be detected in the recombination zone by a detector, and where, the beam receiver comprises a transparent housing which, in the form of a beam profile splitter encloses at least two transmission elements wherein, before beam recombination, changes in the time-domain radiation characteristics in the separate and preferably parallel split beams can be implemented, namely different delays in reflection or transmission, phase changes and/or changes in optic path lengths. This apparatus is characterized by not requiring moving parts.
The invention offers the advantage that at least two transmission elements designed as beam splitters may be controlled independently of one another and that as a result a plurality of control modes is feasible.
In another aspect of the invention, one transmission element may generate a reference split beam or leave it unchanged and be mounted parallel to at least one further transmission element that may control the time-domain characteristic of the other split beam(s). Claim 14 specifically provides that the other split beam(s) can be phase shifted by the further transmission element, preferably so as to be 180xc2x0 out of phase with the first split beam, so that specular or interference effects can be produced in simple manner.
Moreover the invention relates to apparatus to control and measure electromagnetic radiation by combining geometric beam splitting and ultra-narrow pulses of less than 1 ns, preferably less than 100 ps, in such a way that at their output sides split beams running along at least two optic paths of different lengths or phase changes are detected before reaching or when reaching a recombination zone by means of a detector or other measuring device. This feature allows new applications, illustratively pulse idiosyncracies using a nonlinear crystal or a two photon absorption detector (TPA), moreover the generation and measurement of pulse patterns, distance or chirp measurements, phase measurements or phase distortions, characterizing and analyzing materials, polarization adjustments etc.
In that respect, the invention provides that a test object can be inserted between the beam receiver and at least one beam profile splitter or between at least one beam profile splitter or the recombination device, in order that, for instance, identification or composition of a material can be displayed and/or measured.
A further alternative of the invention is that an object to be tested shall constitute one of the beam profile splitters and/or that two or more objects to be tested shall each constitute one of the beam profile splitters.